1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a thin film transistor (TFT)). The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
Better electric characteristics of a transistor including an oxide semiconductor are required for application to semiconductor devices with higher performance. For example, a transistor having a self-aligned structure, in which a gate electrode and a gate insulating film are formed over a region of an oxide semiconductor film serving as a channel formation region of the transistor, and a source region and a drain region are formed in such a manner that the resistance of a region of the oxide semiconductor film, which is covered with neither the gate electrode nor the gate insulating film, is reduced, is reported (for example, see Patent Document 1). Further, a transistor having a structure in which an oxide semiconductor including aluminum, titanium, or the like as dopant in the depth direction from a top surface is used for a source region and a drain region is reported (for example, see Patent Document 2).